Substantially linear polyesters comprising, as the main constituent unit, an ethylene terephthalate, such as polyethylene terephthalate, have been heretofore used as a material for molded article having various shapes, such as a film, a sheet, a laminate, and a container, because of their excellent stiffness, heat resistance, and gas barrier properties. However, these polyesters are disadvantageous in that the rate of crystallization is slow and they cannot be molded at a high speed. Therefore, a means of elevating the temperature of the mold is used to enhance the rate of crystallization of the polyesters. However, use of this means involves disadvantages in that the resin undergoes thermal deterioration and the molded article tends to be warped.
In order to increase the rate of crystallization and to improve the mechanical properties such as heat resistance and stiffness, of the polyesters, there have been proposed methods comprising blending various additives with the polyesters. These methods include the blending of inorganic compounds such as talc, metal salts of aliphatic or aromatic carboxylic acids, or polymers such as polyalkylene glycols, alkoxypolyalkylene glycols, ethylene-(meth)acrylic acid salt copolymers, unsaturated polyesters, and polyamides with the polyesters. For example, U.S. Pat. No. 3,639,527 proposes copolymers of ethylene or styrene with salts of acrylic and methacrylic acid or the like. U.S. Pat. No. 4,352,904 and U.K. patent application GB No. 2,015,013 propose techniques of blending (1) fillers or the like, (2) sodium or potassium salts of organic polymers having pendant carboxyl groups, and (3) organic esters. Examples of the salts of organic polymers (2) are sodium salts of ethylene-methacrylic acid copolymers and sodium salts of styrene-maleic anhydride copolymers. Japanese Unexamined Patent Publication Nos. 56-145943, 56-127655, and 57-145145 propose techniques of blending the same copolymer salts as those described in (2) above and polyalkylene glycols or their derivatives.
Japanese Unexamined Patent Publication Nos. 56-109244 and 56-109245 propose a technique of blending polyethylene wax, optionally a nucleating agent and a crystallization promoter with a polyethylene terephthalate resin. Similarly, U.S. Pat. No. 3,405,198 discloses the technique of blending polyethylene.
Japanese Unexamined Patent Publication No. S 58-76446 and 57-8241 propose techniques of blending specific polyethers or polyesters. UK Patent Specification GB No. 1282679 and Japanese Unexamined Patent Publications Nos. 51-148744, 57-38847 and 58-63743 propose a technique of blending metal salts of carboxylic acids.